1. Field of the Invention
This invention relates to an image signal recording and reproducing system which records an image signal on a recording medium and reproduces the recorded signal from the recording medium.
2. Description of the Related Art
Among known image signal recording and reproducing systems, a system called a still video system has been arranged, for example, to record a still image signal on a recording medium such as a magnetic disc and to reproduce the recorded still image signal from the medium.
Other image signal recording and reproducing systems such as video tape recorders and the like are now tending to become capable of giving a higher picture quality. In the case of the still video system, the picture quality of the still image signal has come to be recorded and reproduced with a higher degree of picture quality by changing the conventional recording format to a so-called high-band recording format. In accordance with the conventional format, a luminance signal which includes a synchronizing (hereinafter referred to as sync) signal has been recorded after frequency modulating it to have its sync tip part at 6 MHz and a white peak at 7.5 MHz. Whereas, in the high-band format, the sync tip part is set at 7.7 MHz and the white peak at 9.7 MHz to broaden the frequency modulating deviation range from 1.5 MHz to 2 MHz.
In addition to the function of reproducing the recorded still image signal from the magnetic disc and displaying it on a monitor device, the still video system has another function, which is as follows: The reproduced still image signal is digitized. The digitized signal is first stored in a memory. After that, the stored signal is read out to be transmitted to a remote place through a telephone line or the like or to be reproduced in the form of a hard copy by means of a printer.
FIG. 1 of the accompanying drawings shows in outline the arrangement of the conventional device for storing in a memory the still image signal reproduced from the magnetic disc. Referring to FIG. 1, the still image signal reproduced from the magnetic disc which is not shown is supplied to a sync signal separation circuit which is not shown. The sync signal separation circuit then separates from the still image signal a composite sync signal which consists of a horizontal sync signal and a vertical sync signal. After that, the still image signal is converted into three primary color signals R, G and B by an image signal processing circuit which is also not shown. One of the three primary color signals, say, the signal R is supplied from an input terminal 53 to a clamp circuit 54. The clamp circuit 54 performs a clamping process on the signal R. After the clamp circuit 54, the signal R is supplied to an analog-to-digital (hereinafter referred to as A/D) converter 55.
Meanwhile, the composite sync signal separated by the sync signal separation circuit is supplied via an input terminal 59 to a gated oscillator 58. The gated oscillator 58 then generates a sampling clock signal in synchronism with the horizontal sync signal included in the composite sync signal. In accordance with the sampling clock signal generated by the gated oscillator 58, the A/D converter 55 digitizes the signal R supplied from the clamp circuit 54 and supplies it to a memory circuit 56.
A memory control circuit 57 is arranged to control the writing and reading actions of the memory circuit 56. In accordance with the composite sync signal separated by the sync signal separation circuit and the sampling clock signal generated by the gated oscillator 58, the memory control circuit 57 applies a control signal to the memory circuit 56 for every horizontal period. This causes the signal R supplied to the memory circuit 56 to be stored at a desired address on the memory circuit 56 for every horizontal period.
While the high-band recording format enables the conventional still video system to enhance the degree of resolution of a reproduced still image by broadening the recording frequency band of the luminance signal, the reproduced still image is apt to be affected by a jitter component resulting from uneven rotation, a deformation or an eccentric state of the magnetic disc. This has caused the picture quality deterioration of the reproduced still image to be manifest to the visual sensations.
Further, in the conventional still video system, the edge part of the horizontal sync signal has sometimes been distorted by the frequency modulation and demodulation processes performed for recording and reproduction as shown in FIG. 2(a). In that event, the waveform of the horizontal sync signal separated by the sync signal separation circuit becomes as shown in FIG. 2(b). The waveform distortion in the edge part of the horizontal sync signal causes a deviation of the time base of the horizontal sync signal to an extent as shown by .DELTA.t in FIG. 2(b).
In the event of occurrence of the above-stated time base deviation in the edge part of the horizontal sync signal, the picture quality deteriorates, because: The device shown in FIG. 1 is arranged to have the sampling clock signal generated by the gated oscillator, 58 on the basis of, for example, the falling edge of the horizontal sync signal included in the composite sync signal supplied; to A/D convert the signal output from the clamp circuit 54 in synchronism with the sampling clock signal thus generated; and to store the A/D converted signal in the memory circuit 56. In this case, there arises a discrepancy in time among the sampling points obtained at the A/D converter 55 for every horizontal scanning line. Further, since the horizontal address deviates for every horizontal scanning line when the signal is stored at the memory circuit 56, a still image signal which is read out from the memory circuit 56 and converted into an analog signal by a digital-to-analog (D/A) converter (not shown) comes to show some discrepancy in the vertical edge part of the image.
Further, in the still video system, the resolution of the still image recorded is determined almost solely by the number of picture elements of an image sensor (such as a CCD image sensor) which is used for forming the still image signal. More specifically, the horizontal resolution is determined by the number of picture elements in the horizontal direction of the image sensor and the vertical resolution by the number of scanning lines of the current NTSC system.
To enhance the resolution of the still image, therefore, a still video camera has been arranged to use two image sensors each of which has 500 (vertical).times.1,200 (horizontal) picture elements (hereinafter referred to as first and second image sensors). The first and second image sensors are arranged on the image sensing plane of the photo-taking optical system of the still video camera in such a way as to have the position of each picture element of one of the first and second image sensors deviate to an extent corresponding to 1/2 picture element from that of the other in the vertical direction, as shown in FIG. 3. The image is sampled by reading out the signal stored by picture elements indicated by a mark ".smallcircle." in FIG. 3. Image signals output from the first and second image sensors are thus formed by sampling the signals output from picture elements which are disposed in every other picture element position in the horizontal direction. In addition to that, the sampling points of the first image sensor deviate from those of the second image sensor in the horizontal direction to an extent corresponding to one picture element. The image signals output respectively from the first and second image sensors are recorded in four tracks on a video floppy disc by means of two heads. The recording performed on the video floppy disc in this manner results in a recording pattern as shown in FIG. 4. Referring to FIG. 4, an image signal which corresponds to an odd-number field period and which has been output from the first image sensor is recorded in a track A. An image signal which corresponds to an odd-number field period and which has been output from the second image sensor is recorded in a track B. An image signal which corresponds to an even-number field and which has been output from the first image sensor is recorded in a track C. An image signal which corresponds to an even-number field and which has been output from the second image sensor is recorded in a track D.
In reproducing the record, reproduced image signals obtained from the four tracks of the video floppy disc are composed into an image at an image memory. Then, on the image memory, an interpolation process is carried out to obtain a still image signal of approximately 1,200 (horizontal).times.1,000 (vertical) picture elements. This image signal is then supplied to a video printer or a high-definition TV monitor.
The image signal which output from the image sensor is thinned out as mentioned in the foregoing. After that, the image signal is recorded on a video floppy disc. The recorded image signal is reproduced from the video floppy disc and is then stored in the image memory to be recomposed into a still image. The recomposition of the still image is carried out by a sort of analog transmission of sampled values. The analog transmission requires strict control over the overall amplitude and phase characteristics of a transmission route consisting of a magnetic recording-reproducing system, frequency demodulation system, etc.
Further, in order that the reproduced signal is performed at timing accurately corresponding to the positions of sampling points shown in FIG. 3, it is necessary to use a time base variation correcting circuit for accurately correcting time base variations taking place in the recording-reproducing system. For this purpose, the following method is conceivable: In recording, a pilot signal for correcting time base variations is recorded by frequency multiplexing between a frequency-modulated color-difference signal band and a frequency-modulated luminance signal band as shown in FIG. 5. Then, in reproducing, the time base variations are corrected by forming a clock signal having the same jitter component as the reproduced pilot signal by means of a PLL circuit and by using this clock signal for writing into the above-stated image memory.
Meanwhile, the S/N ratio of reproduction becomes better accordingly as the recording level of the pilot signal is higher. A high recording level of the pilot signal thus permits the time base variation correcting action to be stably carried out to give a less amount of remnant jitter. However, an excessively high recording level of the pilot signal would cause a leak of the pilot signal into the frequency-modulated color-difference signals or frequency-modulated luminance signal; or a cross-modulation distortion relative to a frequency modulating carrier signal. This would deteriorate the quality of the reproduced image. To avoid this, therefore, the maximum recording level of the pilot signal has been set at such a level that would not have any adverse effect on the frequency-modulated image signal.
However, the reproduction output of the video floppy disc obtained from one recording track differs from the output obtained from another track. If the recording level of the pilot signal is fixed at a level, it brings about the following problem: If the recording level of the pilot signal is set at a level most suited for the innermost track of the video floppy disc which is most prone to the adverse effect of the cross modulation with the frequency modulating carrier signal, the phase of the pilot signal would be affected, during reproduction, by the lower side wave of the frequency modulated luminance signal recorded in a track located on the outer side of the floppy disc. This would hardly permit stable correction of time base variations. Then, an attempt to avoid the adverse effect of the lower side wave of the frequency-modulated luminance signal or the like on the S/N ratio of the pilot signal by setting the recording level of the pilot signal at a higher level would result in an increase in distortion due to the cross modulation with the luminance frequency modulating carrier signal recorded in the inner track. Then, the picture quality would also be deteriorated by occurrence of a moire in the reproduced image.
The video floppy disc is arranged to be rotated at a fixed speed. Hence, the speed of a magnetic head relative to the video floppy disc varies every time its position shifts from one track over to another. As a result, a reproduced signal obtained from an inner track has a lower level and a poorer S/N ratio than a reproduced signal obtained from an outer track.
Generally, the dispersed degree of the phase error of the pilot signal supplied to the PLL circuit is inversely proportional to its S/N ratio (power ratio) and proportional to the loop noise band of the PLL circuit. If the loop noise band is unvarying, the phase error increases accordingly as the S/N ratio of the incoming pilot signal decreases and the jitter remaining after correction of the time base variations also increases. Therefore, the time base variation correcting accuracy of the PLL circuit is lower for the tracks located on the inner side than the accuracy for the tracks located on the outer side. To solve this problem, it is conceivable to narrow the loop noise band by setting the loop band of the PLL circuit at narrower band in such a way as to have an allowable degree of remaining jitter for the innermost track. However, this method makes the pull-in range of the PLL circuit narrower. Therefore, with the head at an outer side track during a reproducing operation, when the rotating speed suddenly changes due to a large load on a spindle motor which rotates the video floppy disc, the PLL circuit is incapable of following the sudden change.